Abstract: The random walk model for the existing water quality is improved and a quality 2D random walk model considering the role of self-purification is established based on the assumption of a reaction kinetics of biodegradable pollutants and considering the defects that the water quality model for random walk only takes the convection-diffusion of pollutants into account,and neglects the effect of their own degradation of pollutants.Through the typical case of a model test,the results of the improved random walk model and the 2D convection-diffusion equation analytical solutions are analyzed.It shows that the water quality of the random walk model is consistent with that of the analytical solution.The improved model considers the characteristics of the degradation of water pollutants and visually describes the pollutant migration characteristics.